Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
Power requirements for modern portable electronics are increasing very rapidly; e.g., devices having larger displays, LTE devices (radios, modems, etc.), multi-core processors, and so on. To maintain acceptable up times, such devices utilize batteries with higher capacity. In such systems, battery charging times tend to be very long when conventional power sources are used. The reasons include: (1) limited power capability (USB 5V/1.8A max); and (2) voltage headroom issues between input power source and battery. Furthermore, many readily available power sources (e.g., monitors, notebooks, etc.) cannot be utilized because of their high-voltage operation vs. what the portable device can tolerate. Implementing a solution that requires the use of a secondary portable device connector significantly increases solution and consumer cost (proprietary connector, wall adapter, etc.).
With battery capacities increasing, 5V input voltage does not provide enough voltage headroom to achieve sufficiently high charge currents due to cable, connector, PCB, and charger impedances. Many batteries now have a float voltage of 4.35V which makes this issue worse, especially since the trend is toward the use of higher voltages. For example, a 2S stack provides about 8.4V or 8.7V, thus requiring a voltage higher than 5V to charge efficiently.